I'm a huge fan of the super heavy industrial type stuff (The custom map Hadal is a great example of this) with tons of metal and loose piping and vents, and just generally a much harsher aesthetic.

Beyond that, I like any map that does a good job of portraying that there is a land beyond just the map we're shooting each other in. That always helps with immersion. That's why it was such a big deal that I get the skybox right in Preposterous.

I like ancient architectures like Egyptian, Mayan, Cambodian,....
They feel mysterious and old, also you don't have to follow the "Realistic Rules" because you can build whatever you want in your map, and the explaination is just simply: "Old people is weird, they just like to build stuff like that"

Medieval is a greatly underappreciated theme, both game mode wise and detail wise. You guys remember Helms deep V2? That map was crazy good looking! There was a time, long ago, maybe three or four years ago, when I used to play on a medieval custom maps server that was a ton of fun. It had, like, arena, payload, king of the hill: just a ton of awesome maps. Then, out of nowhere, it shut down, and now the only medieval servers that ever have players are fucking 24/7 degroot servers with like bots for half the players. I would kill to get to see a server with tons of awesome medieval mode maps.

(Note: The only two classes in medieval that have no melee unlocks are Pyro and Engineer. While the Gas Passer seems like a totally fair weapon in this mode, it remains blacklisted. It's unfortunate to see the Pyro still having a pretty pathetic roll in medieval.

idk about the rest of the world, but i love the CTF game mode and the map 2fort and it's style.
CTF is (in my opinion) the perfect game mode, players have a defined front line and a defined work space, for offence classes it gives tremendous incentive to keep pushing the front, defense classes have a stationary place to fortify and have the opportunity to better strategize. support have all-sort of tag-along jobs and if done right it can almost feel like a stealth game to spys.

2fort is basically the unofficial tutorial map of the game.

and the style is just how i have always known TF2 even long before i ever even knew what it actually was.

i know some people will disagree with me.

as for my favorite game mode to map for its...okay its still CTF. but arena is a close second because of how experimental you can get.